Variable voltage drive, particularly for hoisting equipment



April 26, 1949 CHAELCHLIN ETAL 2,468,117 VARIABLE VOLTAGE DRIVE PARTICULARLY FOR HOISTING EQUIPMENT Filed Nov. 29, 1947 Lower/n???- uef ho/sf/n 7Z7r ue I) m m A b S l o/ \E 02 u A 1 Q3 o l J .6 1911.

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|NVENTOR5 h/a/fer- 5 chars/c619 and Patented Apr. 26, 1949 VARIABLE VOLTAGE DRIVE, PARTICULARLY FOR HOISTING EQUIPMENT Walter Schaelchlin, Pittsburgh, Pa., and William R. Harding, East Aurora,

Westinghouse N. Y., assignors to Electric Corporation East Pittsburgh, Pa., a corporation of Pennsylvania Application November 29, 1947, Serial No. 788,922

7 Claims. 1

Our invention relates to variable voltage drives particularly for the operation of hoisting equipment and is hereinafter disclosed with reference to the drawing in which Fig. 1 shows a coordinate diagram of typical speed-torque characteristics, Fig. 2 the circuit diagram of a hoist drive, and Fig. 3 a modification of the drive.

For the operation of cranes and other hoisting equipment, a drooping speed-torque characteristic of the hoist drive, as typified by the curves shown in Fig. 1, is desirable for a smooth and versatile lifting and lowering performance under various load conditions. Conventional electric drives, including variable voltage drives, do not inherently yield a characteristic of this type. According to known proposals, however, such characteristics can be obtained with the aid of especially designed rotating machinery, or by using a special set of control relays in addition to the customary reversing and accelerating contactors and relays.

It is an object of our invention to devise a variable voltage drive which secures the desired drooping speed-torque characteristics by means of a simpler design than heretofore necessary for this type of operation. More specifically, the invention aims at providing a drive which, for securing the drooping characteristic, requires neither especially designed dynamos nor additional relay equipment. It is also an object of the invention to make such a drive capable of rapid and stable response to changes in the adjustment of the master control means so that overshooting and hunting are safely avoided even if the drive operates under a heavy overhauling load. Another object of our invention is to provide a variable voltage drive, especially for hoists, which permits a high-empty-hook speed without requiring for this purpose a generator of more than standard size or extra capacity and high sealing voltage.

In order to achieve these objects, and in accordance with a feature of our invention, we provide a variable voltage drive whose motor speed is basically controlled by varying the field excitation of the generator; and we superimpose on the drive motor a field control with the aid of a rectified alternating current controlled by a saturable reactor in dependence upon the load current of the drive motor.

These and other features of our invention will be apparent from the following description of the hoist control system shown in Fig. 2.

In Fig. 2 the hoist drum of a crane or the like equipment is schematically represented at l. The

drum is driven, usually through a reduction gear (not illustrated), from the armature 2 of a direct current motor M with two separately excited and cumulative field windings 3 and 4. Motor armature 2 is connected in a loop circuit with the armature 5 of a generator G which provides variable armature voltage for the motor. Generator G has a series field Winding 6, a shunt field wind ing '1, and a separately excited control field winding 3, and is driven by an auxiliary motor 9 of substantially constant speed. The motor 9 is shown to be energized from a three-phase alter-: hating current line It and drives also an exciter E which serves as a convenient source of constant direct current voltage. The exciter leads are denoted by H and i2, respectively.

The control field winding 8 of generator G is excited by selectively adjustable voltage from a potentiometric rheostat I3 which is connected across the exciter leads H and I2, preferably through reversing contacts M. The field winding 3 of motor M is also connected across the exciter leads I l and I2, and hence receives normally constant excitation. Motor field winding 4 is output terminals of a recinput circuit is connected across two leads of the alternating current line It) in series with the alternating current or main winding 36 of a saturable reactor 11. Reactor I1 is equipped with a direct current control winding which is attached across a resistor I9 in series with an inductance coil 20. Resistor I9 is series connected in the common armature circuit of generator G and motor M so that the voltage drop developed across the system I9 and effective to control the magnetization of reactor I7 is substantially proportional to the motor load current.

The system may be equipped with the customary protective relays and contactors as well as with other auxiliary relays, contactors and circuit control devices customary in such systems. In particular, it should be understood that the rheostat I3 and reversing contacts M are preferably associated with a master controller and appertaining reversing contactors of customary type, so that the control field winding 8 is energized by voltage and polarity depend on the position of the master controller adjusted by the operator.

Assuming the alternating current line H) to be energized and the auxiliary motor 9 running at the proper speed, the generator G is deenergized when the master controller is in the on position, i. e., when in the schematic illustration the slider of rheostat I3 is placed on the tap contact marked 01f. The motor M is then at rest and a magnetically releasable friction brake (not illustrated) is usually provided to arrest the hoisting drum. When the master controller is placed in the first position, i. e., when the slider of the illustrated rheostat I3 is in engagement with the tap contact marked Pl, a relatively low voltage is applied to the generator field winding 8 so that the motor receives armature current and operates the drum, for instance, in the hoisting direction. The above-mentioned brake is so connected with the master controller that it is released whenever the generator field winding 8 is energized for hoisting or lowering operation. Hence, the motor M will now operate at relatively low speed and with a relatively low torque as typified by the characteristic Hi in Fig. 1. When the master controller is placed in the second position, i. e., when in the illustration, the slider of rheostat l3 engages the contact P2, the excitation of generator field winding 8 is increased so that the motor operates at increased speed and torque, as represented, for instance, by the characteristic H2 in Fig. 1. Similarly, a further increase in the voltage impressed on the generator field winding 8 will cause the motor to operate in accordance with the typical characteristics H3 or H4.

When the load on the motor is overhauling and the resistance controlled by the master controller or rheostat is set for a low value, the motor acts regeneratively and the operating characteristic then corresponds, for example, to curves Ll, L2, L3 or L4 shown in Fig. 1, depending upon whether the voltage applied to generator field winding 8 has low or increased values, respectively. It is assumed that during the above-mentioned hoisting and lowering operations, the condition of the reversing contacts l4 remains unchanged. If the polarity of the voltage across generator field winding 8 is reversed, the motor M is caused to develop torque in the opposite direction, for instance, in order to drive a load in the downward direction with an operating characteristic as typified by curve Ql, Q2, Q3 or Q4 in Fig. 1.

The fact that during the operation of motor M the speed torque characteristics assume the desired drooped and concave configuration apparent from Fig. 1 is due to the load-responsive speed control imposed on the motor M through the variable excitation of field winding 4. When the current flowing in the armature circuit of the variable voltage drive has a low value, the voltage drop across resistor l9 has also a low value so that a correspondingly low magnetization is imposed on the reactor H by the control coil l8. As a result, the reactive resistance of the reactor main field winding 16 is high and the voltage impressed on the motor field winding 4 is correspondingly low so that the constant excitation of field winding 3 is predominant. When the current in the armature circuit of the drive system increases, the voltage applied to the control winding 18 of reactor l'l increases also. As a result, the reactive resistance of main winding it is reduced and the voltage impressed on field winding 4 of motor M is increased. Since the two field windings 3 and 4 act cumulatively, an increase in the motor current has the effect of strengthening the motor field so that the speed of the motor becomes lower than it would be if the motor field'were constant and merely the armature voltage were varied by the field control of generator G. The shape of the motor characteristic is further modified in the desired sense by the presence of the choke coil 20 in the circuit of the reactor control coil l8 and hence can be varied to some extent by changing the impedance rating or magnetizing conditions of the iron core of coil 20.

By virtue of the fact that the automatic control necessary for securing the desired drooping characteristic is accomplished by means of motor field control, the generator can be of standard size and does not require extra capacity and high sealing voltage in order to obtain a high empty-hook speed. The self-energizing shunt fieldwinding i is preferably provided in the generator in order to decrease the voltage burden on the control field coil 8, and hence to decrease the burden on the contacts appertaining to the master control means represented by the rheostat l3 and the contacts 14. The provision of a separately excited shunt field has also the advantage ofincreasing the time constant of the variable voltage system, and hence avoids the occurrence of excessive current surges when the controller is moved too rapidly, for instance, from the off position to the full speed position. In order to secure these advantages, the polarity of connection of the shunt field winding 1 must be such that the fields of windings I and 8 are cumulative. For reducing the circulating current when the controller is in the off position and the abovementioned brake is set, a series field, as represented by field winding 6, may be added and the field of this winding should be difierential relative to that of the control field winding 8.

The fixed excitation applied to the field winding 3 of motor M is rated to furnish an appreciable portion of the total field excitation necessary for operation at rated full speed of motor M. For instance, a rating of field winding 3 for 40% of this total excitation is satisfactory.

When hoisting and lowering an extremely light load, for instance, during empty-hook operation, the voltage impressed on the control coil l8 of reactor H is extremely small or negligible. Consequently, the motor field is then provided virtually only by the field winding 3. As a result, the empty-hook speed can be kept very high, for instance between 200 and 250% rated full speed. When hoisting full load, the armature current of the motor is a maximum. The voltage across resistor IQ is then also a maximum so that the reactor I1 is saturated and offers minimum reactive resistance in its main winding l5. Consequently, the field winding 3 receives maximum excitation and the motor operates with a maximum field. When lowering full load, the motors operates as a series generator. Under such conditions, there is normally the danger that the motor overshoots and starts hunting. In systems according to the invention, however, the lowering operation remains stable because the mechanical time constant of the motor is relatively short with respect to the time constant of the reactor control winding, the latter time constant being adjustable, for instance, with the aid of the inserted inductance coil 20. In other words, there is no overshooting or hunting because the change in motor speed can very rapidly follow the variations of the current impressed on the motor field winding 4.

Since the inductance coil 20, the coils of the reactor I1, and the rectifier 15 are traversed by small currents as compared with those effective in the armature circuit of the drive, these cirgenerators of special type as well as an additional set of control relays.

The embodiment shown in Fig. 3 differs from the drive system of Fig. 2 by a modification of the motor field circuits, and the illustrations is limited to representing substantially only the modified portion, all non-illustrated circuits being assumed to be as represented in Fig. 2, According to Fig. 3, the motor windings 3 and 4 of, for instance, the same number of turns and the same resistance. The two windings are connected in non-adjacent branches of a Wheatstone bridge network. The two intermediate branches contain resistors 2| and 22, respectively. Two diagonally opposite terminals of the network are connected to the constant volt age leads H and I2, respectively. The other two terminals of the network are connected to the output side of the rectifier l5. During the operation of the drive, both field windings excitation from rectifier IS, the latter being additive to the constant component excitation and foregoing the above-mentioned advantages afiorded by the invention.

We claim as our invention:

1. A variable voltage drive comprising a direct current resistance means, said generator field winding being connected to said direct-current supply means under control by said resistance means to receive adjustable excitation, one of said two circuits being connected to said direct-current supply means to receive normally constant exmotor, and a rectifier interposed between said reactor main winding and said alternating-current supply means and connected to said other trolling said voltage, direct-current circuit means, adjustabl resistance means, said generator field winding being connected to said circuit means under control by said resistance means to receive adjustable excitation, one of said motor field windings being connected to said circuit means to receive normally constant excitation, alternating current supply means, a saturable reactor device having an alternating-current main winding connected to said supply means and having a direct-current control winding for controlling the reactance of said main winding, said control winding being connected to said armature circuit to be energized in dependence upon the current flowing in said armature circuit, and a rectifier interposed between said energizing it in dependence upon the current flowing through said main winding.

separately excited field winding.

4. A variable voltage drive according to claim 3, comprising an additional field winding on said generator series connected in said armature circuit and disposed for difierential action relative to said separately excited field winding for reducing the circulating current in said armature circuit when said separately excited field winding is deenergized.

trolling said voltage, direct-current circuit means, adjustable resistance means, said generator field said other motor field winding.

6. A variable voltage drive comprising a direct under control by said resistance means to receive adjustable excitation, one of said two circuits being connected to said circuit means to receive normally constant excitation, alternating-current supply means, a saturable reactor device having an alternating-current main winding connected to said supply means and having a directcurrent control winding for controlling the reactance of said main winding, a resistor seriesconnected in said armature. circuit, an inductance coil, said control windin of said reactor being series-connected with said coil across said resistor, and a rectifier disposed between said reactor main winding and said supply means and connected to said other circuit for energizing it in dependence upon the current flowing through said main winding.

7. A variable voltage drive comprising a direct current drive motor having an armature and field means, said field means having two circuits for imposing two cumulative voltages respectively on said field means, a generator having an armature connected to said motor armature to provide variable voltage therefor and having a field Winding for controlling said armature voltage, controllable direct-current means connected to said field winding for providing controllable excitation therefor, alternating-current supply means, a katurable reactor having a main winding connected to said alternating-current supply means and having a control winding for controlling the reactance of said main winding, said control winding being connected to said motor armature to be energized in dependence upon the load current of said motor, a rectifier interposed between said main winding and said alternating-current supply means and connected to one of said cir cuits for energizing said one circuit in dependence upon the current flowing through said main winding, and direct-current supply means of normally constant voltage connected to said other circuit.

WALTER SCHAELCHLIN. WILLIAM R. HARDING.

No references cited. 

